In recent years, in various fields, a system has been used in which a plurality of communication devices are connected and the communication devices are each assigned a function, exchange data with one another and perform various processes in cooperation. In the communication system, a noise generated on a communication medium connecting the communication devices affects the communication quality. Therefore, it is necessary to take measures for preventing the noise or to realize communication so as not to be affected by the noise.
In the field of in-vehicle LANs (Local Area Networks) disposed in vehicles, by using ECUs (Electronic Control Units) which are communication devices and causing the ECUs to perform specialized processes to exchange data with one another, various functions are implemented as a system. Vehicle control has been shifted from mechanical control to electric control and the functions of the ECUs are specialized to increase the number of functions implemented in the system. Consequently, the number and type of communication devices increase and the number of communication lines (in-vehicle harness) connecting the communication devices also increases. Moreover, an increase in the amount of data transmitted and received in the communication system makes it necessary to transmit and receive large amounts of data faster.
In the field of vehicles, it is particularly desired to improve fuel economy by reducing the vehicle weight or to reduce driving power. It is also desired to improve passenger comfort by increasing in-vehicle space. Therefore, it is required to reduce the weight of the in-vehicle harness by reducing the number of communication lines used for the in-vehicle LAN.
For this reason, PLC (Power Line Communication) in which a carrier wave for communication is superimposed on an existing power line to realize communication is drawing attention, and its application to in-vehicle LANs has been proposed (for example, see Patent Document 1). By applying PLC to in-vehicle LANs, saving of lines is realized, which enables the reduction in the weight of the in-vehicle harness.    Patent Document 1: Japanese Patent Application Laid-Open No. 2006-067421